MAL-6(2, 2,6,6,-tetra-methyl-4-malaimide-piperidin-1-oxyl) was used as spin label in studying the effects of ionizing radiation on rat erythrocyte membrane protein. By this approach, It was found out: (1). 100-400 Gy irradiation in vitro gave no obvious effects on ESR spectra revealed by spin label-SH group in the erythrocyte membrane. (2). Whole-body irradiation with 8.5 Gy caused increase of w/s ratio for the erythrocyte membrane.,It was suggested that the above-mentioned results might be attributed to the damage action of the endogeneous active oxygen species produced from imbalance between their generation and scavenging in erythrocyte or living body.